Vapor diffusion coating process



y 2, 1963 R. B. PUYEAR 3,096,160

VAPOR DIFFUSION COATING PROCESS Filed June 19, 1961 2 Sheets-Sheet 1 [4.5 MlLS IN VEN TOR.

ROBERT B. PUYEAR kw Film A T TORNE V y 1963 R. B. PUYEAR 3,096,160

VAPOR DIFFUSION COATING PROCESS Filed June 19, 1961 2 Sheets-Sheet 2 10.0 MILS 4.2 MlLS IN VEN TOR.

ROBERT B. PUYEAR Mr R PM A T TORNE V United States Patent 3,096,160 VAPOR DIFFUSIGN COATING PROCE S Robert B. Puyear, Kokomo, Ind., assignor to Union Carbide Corporation, a corporation of New York Filed June 19, 1961, Ser. No. 117,974

16 Claims. (Cl. 29-497) This invention relates to the application of difiused metallic coatings to metal articles.

Engineering parts subjected to high temperatures and corrosive atmospheres can often be made more resistant' to these forces by the application of a protective surface coating to the part. The protective coating composition is selected to give increased resistance to those forces which attack the surfaces of an article, namely oxidation, corrosion, and erosion, just as proper metallurgical design selects the alloy base of the article on the basis of its tensile and creep strengths. Additionally the coating must be adherent and ductile enough to permit the intended use of the coated part without danger of spalling or chipping of the coating. Cementation processes such as chromizing and calorizing produce coatings of chromium and aluminum, respectively, which are resistant but which often have a brittle bonding layer with the base metal.

Protective coatings composed of alloys are very desirable, in view of the superior properties possessed by many alloys over the elemental metals. Of exceptional utility as a protective coating would be a difiusion coating of certain aluminum-containing alloys and/or intermetallic compounds of iron and cobalt which are reported to possess exceptional oxidation resistance as well as the ductility and strength needed for a good bond. These materials further possess a resistance to high temperatures greater than that of the individual constituents of the materials.

Present methods for producing alloy coatings on metallic articles generally require a multiple step operation with a coating of one metal followed by the coating with the other metal, and then a heating to achieve the alloying. Such processes are complicated and unwieldly, and do not consistently yield the desired coating composition or thickness. It is especially difiicult to coat articles having complex shapes by electrodeposition. The part of the article nearest the sacrificial electrode receives the heaviest coating and the crevices, internal cavities, and corners receive little or no coating.

It is the primary object of this invention, therefore, to provide compositions and methods for producing adherent diffusion coatings of high temperature resistant aluminum alloys.

It is another object of this invention to provide a method for applying aluminum alloy difiusion coatings on metallic articles which method is simple and requires a minimum of steps.

"ice

It is also an object of this invention to provide an aluminum alloy diffusion coating for metallic articles giving maximum oxidation and erosion resistance to the coated article, thereby allowing its use in more stringent applications than ordinarily possible.

Other aims and advantages of the invention will be apparent from the following description and the appended claims.

In accordance with these objects a process for diflfusion ding an article to be coated in particulated, prealloyed charge material consisting essentially of from about 19 percent to about 35 percent by weight aluminum and the balance at least one metal selected from the group consisting of iron and cobalt and incidental impurities, and heating said charge and contained articles in the presence of at least one material selected from the group consisting of ammonium halides and aluminum halides, said heating being conducted with the exclusion of air at a temperature between about 1400 F. and 2000" C. for a time sufiicient to form a coating.

In the drawings:

FIG. 1 is a cross-section, magnified 500 times, of the diffused coating produced on a cobalt-base article by the process of this invention;

FIG. 2 is a schematic representation of a reaction chamber or retort for containing the charge material, articles to be coated, and carrier material according to the process of this invention;

FIG. 3 is a cross-section, magnified times, of the diffused coating produced on an iron-base alloy by the process of this invention;

FIG. 4 is a cross-section, magnified 150 times, of the diifused coating produced on the iron-base article of FIG. 3 but with another coating thickness.

Articles that can be coated according to this process include alloys having a base of a metal selected from the group consisting of iron, nickel, cobalt, and copper.

Preferably the articles to be coated are selected from those iron-base, nickel-base, cobalt-base and copper-base alloys which contain a predominant amount of the base metal. In this regard, predominant means possessing that minimum amount or iron, nickel, cobalt or copper which will result in the superior coating thickness and characteristics shown in the drawings and the examples following. As used herein, the term alloys is meant to include the pure or substantially pure metal itself, i.e. iron, nickel, cobalt or copper articles.

In Table 1 are shown a number of representative metals and alloys which were satisfactorily coated by the process of this invention. The compositions do not show the carbon content or other minor constituents, except that manganese and silicon contents are shown for the iron-base alloys.

TABLE 1 Nominal Composition of Metals and Alloys Coated Alloy No.

00 Mo Cr Others Form Iron-base Alloysz wil coating metallic anticles is provided comprising embed- TABLE L-Continued Nominal Composition of Metals and Alloys Coated AlloyNo. Ni co Ie M or W Mn Si Others Form Wrought.

o. Wrought. 5. Al5Tl- Cast.

Cu Wrought.

The charge material is a crushed or shotted prealloyed aluminum alloy compound of'the indicated materials. More specifically the charge material includes alloys of from about 19 to about 35 percent by weight aluminum and the "balance substantially all iron and incidental impurities; alloys of from about 19 to about 35 percent by weight aluminum and the balance substantially all cobalt and'incidentalinipurities; and alloys of from about 19 to -about'35 percent aluminum and the balance substantially all iron and cobalt and incidentalimpurities. Specific and preferred chargematerial compositions are percent aluminum-70 percent iron including incidental impurities and 30 percent aluminumpercent cobalt- 35 percent iron including incidental impurities. p

The charge material is of a size passing a No. 2 mesh sieve and held on a No. 50 mesh "sieve. The preferred charge material' consists of that material passing a No. 4 mesh sieve and held on No. 20 mesh sieve-the sieve sizes being those of the UnitedSta-tes Standard Screen Series. With the use of these large particle sizes there isno problem of sintering of the charge material to the article to be coated as occurs when smaller sized charge material particles are used;

A chemical material, included in the retort with the charge material and article tobe coated, is designated a carrier material for the purposes of this description because it is believed to bring the coating elements to the surface of the article to be coatedQ This material may be one or more of the halides of ammonium or aluminum. Preferred carrier materials are 'aluminum'chloride, ammonium fluoride, ammonium chloride, ammonium bromide, and ammonium iodide. Aluminum chloride is the carrier material used inthe preferred embodiment of the 2 invention with the preferred charge material of 30 percent aluminum-70 percent iron alloy. The proportion of carrier material to charge material is from about 0.25 percent to about 3 percent by weight carrier material and from about 97 percent to about 99.75 percent by weight aluminum alloy charge material.

The heating step of the process is conducted'at a temperatu're between about 1400 F. and 2000 F. for a time depending 'onthe thickness ofthe coating intended. The

thickness of the alloy coating can be varied by varying 30 the time orjteniperature of thefhafi lgstepijj In the drawing of the heatingretort, FIG. 2, a heat 7 resistant retort {I1 is} fitted withQahfencirclingchanneEQ shaped flange 12: Cover 13.fits over the retort with its sides 14 in ihegehanna leavingj space 15.. QA suitablejg5 sealing material -16'such-as a fusible silicate or glass isplaced in spaceso that on heating of thefretort, the: I glass melts 'toform a liquid seal allowing the escape of" air from theretort. Contained inith'e retort isthe chargei' material 17 of particulated aluminum alloy in which are embedded several parts 18 to be coated. -Under the charge material is a layer 19 of the selected carrier material. Those parts 20 which have internal cavities 21 to be coated are given an addition of carrier material 19 inside the cavity. Afterthe retortand its contents have been heated and the reaction is complete, the retort is allowed to cool- Y The liquid seal material hardens forminga solid seal preventing the influir of contaminating air into the retort while the contents cool. V

In operation when the retort and its contents are heated, it is believed that the halogen carrier volatilizes driving out the air fromthe retort through the liquid seal; the gaseous carrierreacts with the aluminum alloy charge materialto form volatile metal halides; these compounds decompose at the surface to be coated depositing the coating elements and liberatingthe halogen which is then a free to again enter into the coating process by reacting with the charge material.

The composition and characteristics of the diflfused. coating vary with the composition of the charge material.

By the use of the indicated aluminum alloy charge materials in the-ranges specified, superior coatings are produced. Because of equilibrium conditions set up the retort at the surface of article to be treated and the indicated' charge materialsg ialloy coatings are produced of definite and superior oxidation and erosion resistance.

No filler materials or adsorbents are 'required in the charge material-here, as in other processes. a

Table 2 shows the coatings produced on difierent alloy articles using various charge material alloy compositions. The carrier material was ammonium fluoride and the heating conditions were 16 hours at approximately 2000 F. as shown. The alloy articles were of the same shape and size. p 7 7 TABLE 2.

Efiect of Charge Material Compositions if Asfs'eninfTable 2 the charge material composition must-be kept'within the indicated ranges. A percent aluminum-50 percent iron alloy charge material does not produce a satisfactory coating and in fact results in a corrosive attack by the deposited metal on the article. It is only with the use of an alloy charge material containing from about 19 to about 35 percent aluminum and the balance, in this case, iron that the superior coating cornpositions of this invention are produced.

By means of X-ray diffraction analysis and X-ray fluorescence analysis of the alloy surface coated using the 70 Fe-30 Al charge material it was determined that the surface of the coatings of this invention were composed essentially of the intermetallic compound FeAl. This was true even when coating those nickel and cobalt-base alloy articles which did not contain any iron, thereby, demonstrating that iron and aluminum are co-deposited when the 70 Fe30 Al charge material is used. These FeAl containing coating compounds give the high oxidation resistance and adherent bonding shown in the tables of oxidation and strength tests following. It was also demonstrated by X-ray analysis that there was no FeAl deposited when the charge material composition consisted of other iron-aluminum alloys outside the ranges of this invention. For example, the 50 percent by weight aluminum-50 percent by weight iron charge material produced the irregular coatings noted in Table 2, which coatings contained only aluminum and not FeAl, as demonstrated by X-ray analysis.

The minimum temperature of the heating step is about 1400 F. because at temperatures below 1400 F., the difiusion rate is too low. Actually the minimum temperature is determined by the decomposition temperature of the carrier compounds; and although the ammonium chloride decomposes at 662 -F., and the aluminum chloride sublimes at 352 F., the lower temperature limit is set at 1400 F. for effective coating and diffusion in commerical production processing.

The thickness of the coating produced depends on several variables including time and temperature and these may be varied depending on the intended application. Generally, a heating time of from about 8 hours to 24 hours or more will produce a coating. For example, in FiG. 3, the iron base article was given a 10 mils thick coating by a heating at 1850 F. for 16 hours.

In FIG. 4 the same ailoy was given a 4.2 mils thick coat by reducing the heating temperature to 1600 F. for the same period of time.

In Table 3 the effects on the coating thickness of carrier material, process time, and temperature variations are shown. In each case the alloy charge material consisted of a percent aluminum-70 percent iron composition and the carrier material was as indicated in the proportion of 0.5 percent carrier material and 99.5 percent charge material. The dimensional change resulting from the coating application is generally an increase in dimensions of about one half of the coating thickness applied.

6 TABLE 3Continued Efiect of Time and Temperature Variations on Coating Thickness OTHER ALLOYS Alloy Time, Tempee Carrier Coating Coated hours ature, F. Thickness,

mils

AlCla 1 Continuous heating from room temperature to 2000" F. 2 Constant heating to 2000 F.

As seenin Table 3, the temperature requirements vary according to the desired coating thickness, alloy to be coated, and carrier material used. Table 3 shows the effects of time and temperature variations on the C2 alloy and also shows the rate of coating deposition for various other alloys. With information of this type a specific heating process can be designed to give a coating thickness of from about 1 mil to over 25 mils on a given alloy.

The temperature conditions can be varied as shown in Table 3 to meet a production operation. The heating can be done in a continuous sequence from room temperature to a temperature between about 1400 F. and 2000 F. for the length of time that is equivalent to a heating at a constant temperature between 1400 F. and 2000 F. The heating may also be conducted in stages for several hours at one temperature followed by heating for several more hours at another temperature. The times and temperatures for these operations are governed by the specit' ic operating conditions and need only be at least equivalent to a heating at a temperature between about 1400 F. and 2000 F. for a time suflicient to form an efiective coating.

A preferred heating cycle consists of placing the retort with contained articles and reactants in a furnace having an entry temperature of about 600 F. and an exit temperature of 2000 F. with a travel time of 24 hoursthe temperature being raised from 600 F. to 2000 F. in about 20 hours.

As an example of the practice of the invention, the following description of the preparation of the prealloyed charge material is given. This material may be prepared 7 by several methods, such as powder metallurgical methods, etc., but alloying by fusion is preferred. 'In aninduction furnace melting operation commercially available electrolytic iron (about 99.8 percent iron) and commercially available aluminum shot (about 99 percent aluminum) were used as raw materials in the proportion of 70 percent by weight iron and 30 percent by weight aluminum. The iron was melted first in a magnesia-lined crucible at a temperature 01331301111 2950 F. Aluminum was added to the molteniron in batches of about percent of the total required aluminum to avoid the undesirable effects of an overly exothermic reaction. After all of the aluminum was added, the melt was covered with a slag to. prevent oxidation and volatilization of the aluminum and to permit interalloying. An inert atmosphere blanketmay also be used for this purpose. The alloy was thenshotted, Washed in" water, and dried.

Theshot was then crushed in a jaw crusher to a size passing a' No. 4 mesh sieve and held on a No.- mesh sieve. In other diffusion coating processes a much finer material isused with a resulting dust problem and the possible inclusion of fine'particles in the coating because of sintering. The shot had the actual and intended composition shown in Table 4. The incidental impurities'contained in thejalloy do not interfere with the coating process. Infajct, it must be noted the charge materials need not be made from ultrapure metal sources but rather may be commercially. available iron and aluminur'ngfor example, provided these sources do not contain an impurity content which interferes with the process.

7 TABLE 4 Composition in Weight Percent I Desired I Actual Other compositions of charge materials are made similarly using electrolytic cobalt, or other commercially available forms, when cobalt is part of the prealloyed charge composition.

When the charge material is cobalt-aluminum, the cobalt is first melted and then the aluminum added to the molten cobalt in several small additions.

When the charge material is iron-cobalt-aluminum al- 10y, the cobalt and-iron are first melted together and then the aluminum additions are made. I

It is to be noted that no catalysts, fillers, or adsorbents are used with-the aluminum alloy charge material. A getter, such as titanium, may be used to remove oxygen in which case up to about 2 percent by weight of the total charge material would be powdered titanium metal mixed with the charge material.

The articles to be coated are first cleaned by any of the V usual methods, such as'grit blasting. The articles are then packed within the charge material in the retort with at least enough space between each article to allow for a layer of charge .material. Cavities which are to be coated are charged with carrier material and with charge material.

'The remainder of the carrier material need not be mixed Aluminum 30.0 29. 56 Iron Balance-.- Balance Carbon Low 0.03 Nickel LOW 0.46 Man'nesium LOW 1111 Silicon Low 0.03 Calcium Low nil Chromium Low 0. 03

quickly and volatilizes to purge the system of air and begin the coating action earlier than when the carrier agent is intermixed with the charge material. From about 0.5 to 1 percent by weight carrier material to about 99 to 99.5 percent charge material is generally used.

It is important to note that the articles to be treated need not be composed entirely of the same metal. Composite articles made of various nickel, iron and cobalt alloy parts may be successfully treated by this process. In one example, a jet engine component having parts made of four different alloys was coated in its assembled form. The article was composed of parts of two diflferent cobalt alloys and two different stainless steels as well as various welding alloys. The coating was smooth and continuous over the entire article. V

The loaded retort is heated at the temperature and for the time necessary to give the desired coating on the article to be coated. After the heating cycle is completed, the retort is cooled to a safe handling temperature. Thecoated objects are then removed and washed. 7 They have a smooth, uniform, and continuous surface, even on hard-tocoat articles such as hollow blades and vanes. Additionally the coating on the inner surface of such hollow articles is equivalent in properties to that on the exterior surface.

The crushed charge material maybe reused. In a series of tests a single batch of crushed 70 percent iron- 30 percent aluminum alloy charge material was used ten times and was still usable. Only a washing with water and a screening to remove fine, exhausted particles is required before each reuse. After a large numberof reuses the charge material may change in composition to an extent requiring either replacement or replenishment by remelting the depleted charge material and-adding more pure 7 metal.

Coated and uncoated specimens of the alloys were subjected to oxidation tests as a means to evaluate the improvement of the high-temperature properties efiected by the coatings of this invention. The following oxidation test was used: specimens were weighed and then placed measure of oxidation resistance. Severe weight losses are an indication of heavy oxidation and consequent metal oxide spelling, While large weight gains result from the formation of massive metal-compound scales on the specimen surface. when the specimen undergoes only slight weight changes and little or no spalling.

The'articles to be tested were disc samples about 7% inch in diameter and /4 inch thick with ,5 inch central hole. They were composed of the nickel, cobalt, and iron base alloys indicated in Tables 5 and 6. Some of these articles were coated according to the process of this invention while an equalnumber were left uncoated. The coating was produced by embedding'the articles in a charge material of 30 percent aluminum-70 percent iron alloy with an aluminum chloride carriage agent in the amount of 99.5 percent charge material and 0.5. percent carrier material. The heating was carried outat about 1950 F. for 16 hours.

The oxidationltests were carried out at 2000 .F.- for 500 hours for the nickel and cobalt-base articlesa'nd at 1800 F. 'for 100 hours for the iron-base articles; In.

some cases weighing was made several timesduringthe test as shown in the tables. The weight change is measured as mg. per cm.

Optimum oxidation resistance is indicated TABLE Results of Oxidation Tests at 2000 F. for 500 Hours Weight change, rug/0m."

Alloy No. hrs. 100 hrs. 300 hrs. 500 hrs.

Coated Uncoated Coated Uncoated Coated Uncoated Coated Uncoated +0.82 +0. 72 -41.27 +1.12 -147.40 +1 2 -200 -0.75 +1.18 -19.0 +1.57 50.0 +1.3 -159 +0.42 +0.43 -0.s9 +1.08 3.33 +0.1 -3. Continuous exposure test -11. 2 -372 Continuous exposure test +1. 0 37. Continuous exposure test +2. 7 --60 Continuous exposure test +4. 0 -474 Continuous exposure test -O. 04 3 TABLE 6 Results of Oxidation Test at 1800 F. for 100 Hours Weight change, mgJcm.

Alloy No. 49 hrs. 66 hrs. 83 hrs. 100 hrs CoatedUncoated Coated Uncoated Coated Uncoated Coated Uncoated +5.01 i +215.7 +5.86 I +2380 +0.02 +2309 +7.5 +2024 Continuous exposure test +4. 1 203. 7 +2.40 1 -19. 29 1 +1.04 1 .03 1 +1.55 -19.ss +1.1 -20.8 Continuous exposure test +3. 7 l, 9 +1.69 l 1.s0l +1.54 1 -2.02 +1.00 2.45 +1.5 +3.3 Continuous exposure test +4. 7 18. 4

The oxidation test results of Tables 5 and 6 show that In an even more severe test the specimens were conboth the coated and the uncoated specimens first under- 'tinuously exposed to an oxyacetylene flame for three 3- went a weight gain as an intial layer of oxide formed. hour periods. A temperature of about 2050 F. was Spalling soon occurred on the uncoated specimens and maintained on the specimens in this test with the weight net weight losses resulted, while the oxide layers on the 40 change of the specimen used as the measure of the excoatcd specimens were more protective and the rate of tent of the erosion. The coated specimens were coated attack was at every low level. 1n the same manner as in the previous tests. The results In another test coated and uncoated specimens of a. of th1s test are shown in .Table 7 where it is seen that the C2 alloy article were heated to a temperature of 2300 uncoated specimens exhibited ametal loss while the coated F. in dry air. At the end of 4 hours, the uncoated specispecimens developed a protective oxide film. mens had lost an average of 141 rug/cm. whereas the coated specimens were unchanged. Further, at the end of TABLE 7 20 hours the coated specimens had gained only about 1.2 mg./cm. Flame Erosion Test Results These tests demonstrate that the coatmgs of this mvention are quite effective in reducing the oxidation rate of Weight change,111g./c1n. iron, nickel, and cobalt-base alloys. The tests also show that the coatings are protective at temperatures approach- Alloy 3 hours 6 hours ghours mg the melting points of the base alloy. It 1s to be understood that slnce the coating elements alloy Wlth the base Coated Uneoated Coated Unooated Coated Uncoated metal, the oxldation resistance of a coated article is somewhat dependent on the Characteristics of the base 02 +1.8 +32 +43 metal. For example, it 1s seen that the coated nickel and 02 -fi .1 +3.3 56.2 +4.0 -57.4 cobalt base articles show a greater oxidation resistance than the coated iron-base articles.

A series of thermal shock tests were conducted in a The results of the several oxidation and BIOSiOfl tests special testing device. Specimens of coated and uncoated desol'fbed above demonstrate that the difi-zllsioll coatings articles were automatically rotated to alternately hot and of 113118 invention can be used in applications Where flul'gh cold stations. At the hot stations, the specimens were temps/refines, aggressive environments, y loads, therheated by an oxyacetylene flame for seven seconds to a 11131 Phock, high Velocity gas stream make th 00 temperature of about 1950 P. Then at the cold stations "Fntlonal alloys unsuitable r use Or Where coated arthe specimens were cooled in a blast of compressed air for holes of inexpensive Y can be used in Place Of Solid seven seconds to about 300 F. The coated and uncoated bodies of expensive, highly alloyed materialsspecimens were of Alloy Iflo. C2. Four uncoated speci- Additional tests were made to determine the mechanical mens and four coated speclmens were tested. After each properties of the coatings and to determine the effect of 50 cycles of dthe above-described test, the specimens were the coating process on the mechanical properties of the iemovedkgn Tixammed at 12d diameters magn fication base alloy. Short time tensile tests were made on coated co; fir-act 2:00 eluncoalteld artllciles showed considerable and uncoated specimens of Alloy No. C3 as shown in Slrl cwsda clysc efst anzocgss w do the coated specimens I' able 8. The coated specimens were coated as before 0 6 H0 0110 a or cycles. and tested as standard inch tensile specimens.

TABLE 8 Average T enszle Test Results Test Ultimate Yield Elonga- Reductempera- Condition tensile strength tion in tion of ture, F. strength, 0.2% Ofi- 1 inch, area,

p.s.i. set, p.s.i. percent percent Room Coated- 93. 500 70, 400 17. 1 9.8 Uncoated- 108, 000 76, 200 9. 11.0 1, 200 C0ated 61, 400 36, 000 11. 0 l2. 9 Uncoated. 75, 500 12.0 12. 6 l, 500 Coated 52, 500 32, 400 16. l 18. 7 Uncoated 63, 200 l5. 0 18. l, 700 Coated 43, 500 29, 100 20. 4 26.0 Uncoated- 34, 100 22. 0 35. 8 1, 800 Coated 34, T00 20, 800 31. 3 40. 9 Unc0ated 29, 400 31. 5 38: 8

It is seen from the results of Table 8 that the coating had a relatively small influence on the tensile properties. The ultimate strength is lower than that of the uncoated alloy at 1500 F. and below. At higher temperatures, the ultimate strength of the coated specimens was slightly above the typical values of the uncoated alloy. The elongation of the alloy was not significantly affected by the coating. Generally, however, the yield and ultimate tensile strengths of the coated alloy are lower than those of the uncoated specimens of the same alloy while the elongation of the coated alloy is higher.

Char-py impact tests were conducted on coated and uncoated specimens of Alloy N0. C2 at several temperatures using unnotched bars. Table 9 shows the results of these tests.

TABLE 9 Charpy Impact Strength of Coated and Uncoated Bars 7 Charpy impact strength, it.-lb. Test temperature, Condition 7 High Low Average Room Uncoated- 26 18 23 Coated--- 32 12 22 1200 Uncoated- 77 39 62 Coated 82 38 58 1600 Uncoated- 64 43 56 Coated 60 38 46 1800 Uncoated- 54 48 52 Coated 52 35 40 The results of these'tests show that the coated specimens generally exhibit a loWer-impact strength 'than the uncoated specimens but that the difference is relatively minor. More important is the fact that the coatings resisted spalling, cracking, and chipping under the most severe conditions of impact and deformation. In both the tensile tests and the impact tests conducted at temperatures of 1400 F. and above, the coating remained intact on the surface of the specimen, deforming with the base metal, even at the area of rupture. There was no spalling, flaking, orstripping ofthe coating, as is common with coating applied by other methods. This is attributable to thecompatible ductilities of the coating and the base alloy. Additionally the coating is thoroughly diffused into'the base object and 7 to attack by oil ash containing sulfur and vanadium compounds showing the usefulness of the coatings on hardware in furnaces burning residual fuel oils and in other similar applications.

."It is seen that the coatings of this invention provide protection to metal particles-from high temperature oxidation aosaieo and erosion attack, as well as thermal shock and mechanical fatigue and that the coating process does not significantly aifect the strength of the base alloy but rather makes the base alloy suitable for use in more aggressive environments than originally possible. In addition, the high strength, adherence, and ductility of the coating al lows for a thoroughly diifused and compatible bond with the base metal.

The above-described structural tests were conducted on iron, nickel and cobalt articles only, but oxidation tests were conducted on the diffusion coated copperM4 article shown in 'llable 4. The results of these tests are presented iu-Table l0. 7

TABLE 10 R sults of Oxidation of M4 Alloy [Same sample tested at each temperature] Unit weight change, Appearance Temp., Time, mg./c1n.

F. hours Coated Uncoated Coated Uncoated 4 0.32 l3. 7 Retained Discolored 1,200 luster. dull oxide scale 1,400 3 +0. 26 59. 0 Retained Dull oxide luster scale. 1,600 3 +0. 70 106.6 Retained Dull oxide luster. scale. 1.800 3 +3. 66 206. 5 Slight dis- Heavy dull coloration. oxide scale.

7 and the balance substantially all at least one metal se-.

' parts, when uncoated, must be serviced or replaced periodically because ofdeterioration of the metal resulting from oxidation and subsequent scaling. The coating of this invention-improves the oxidation resistance of these parts to a remarkable; degree; consequently, the parts are less likely to fail or require periodic replacement.

The coating of this invention 'when applied to copper and copper-base alloys has a yellow color similar to pure gold. The combinedproperties of tarnish and oxidation resistance iandapleasing appearance makes this coating suitable for use on marine hardware and fixtures, decorative and other applications where an aesthetic quality is desired. For specific applications, a copper alloy with the desiredphysical and mechanical properties could be coated, thereby providing a novel article of manufacture.

a What is claimed is:

1. A method for difiusion coating metallic articles comprising embedding the article to be coated in particulated, prealloyed charge materialconsisting essentially of from about 19 percent to about 35 percent by weight aluminum lected from the group consisting of iron and cobalt and incidental impurities, and heating said charge material and contained articles in the presence of at least one carrier material selected from the group consisting of ammonium halides and aluminum halides, said heating being conducted with the exclusion of air at a temperature between about 14009 1 and 2000 F..for a time suificientto form the coating.

'2. A method for difiusion coating metallic articles composed of a material selected from the group consisting of iron-base alloys, nickel-base alloys, cobalt-base, and copper-base alloys comprising embedding the article to be coated in particulated, prealloyed charge material.

consisting essentially of from about 19 percent to about 35 percent by weight aluminum and the balance substantially all at least one metal selected from the group consisting of iron and cobalt and incidental impurities,

13 and heating said charge material and contained articles in the presence of at least one carrier material selected from the group consisting of ammonium halides and aluminum halides, said heating being conducted with the exclusion of air at a temperature between about 1400 F. and 2000 F. for a time suflicieut to form the coating.

3. A method for difiusion coating metallic articles composed of a material selected from the group consisting of iron-base alloys, nickel-base alloys, cobalt-base alloys, and copper-base alloys comprising embedding the article to be coated in particulated, prealloyed charge material consisting essentially of from about 19 percent to about 35 percent by weight aluminum and the balance substantially all iron and incidental impurities, and heating said charge material and contained articles in the presence of at least one carrier material selected firom the group consisting of ammonium halides and aluminum halides, the carrier material being present with the charge material in the relationship of from about 0.25 per cent to about 3 percent by weight carrier material to from about 97 percent to about 99.75 percent charge material, said heating being conducted with the exclusion of air at a temperature between about 1400 F. and 2000" F. for a time sufficient to form the coating.

4. A method for difiusion coating metallic articles composed of nickel-base alloys comprising embedding the article to be coated in particulated, prealloyed charge material consisting essentially of from about 19 percent to about 35 percent by weight aluminum and the balance substantially all iron and incidental impurities, and heating said charge and contained articles in the presence of at least one carrier material selected from the group consisting of ammonium halides and aluminum halides, said heating being conducted with the exclusion of air at a temperature between about 1400 F. and 2000 F. for a time suflicient to form the coating.

5. A method for difiusion coating metallic articles composed of cobalt-base alloys comprising embedding the article to be coated in particulated, prealloyed charge material consisting essentially of from about 19 percent to about 35 percent by weight aluminum and the balance substantially all iron and incidental impurities, and heating said charge material and contained articles in the presence of at least one carrier material selected firom the group consisting of ammonium halides and aluminum halides, said heating being conducted with the exclusion of air at a temperature between about 1400 F. and 2000 F. for a time sufiicient to form the coating.

6. A method for diffusion coating metallic articles composed of copper-base alloys comprising embedding the article to be coated in particulated, prealloyed charge material consisting essentially of from about 19 percent to about 35 percent by weight aluminum and the balance substantially all iron and incidental impurities, and heating said charge material and contained articles in the presence of at least one carrier material selected from the group consisting of ammonium halides and aluminum halides, the carrier material being present with the charge material in the relationship of from about 0.25 percent to about 3 percent by Weight carrier material to from about 97 percent to about 99.75 percent charge, said heating being conducted with the exclusion of air at a temperature between about 1400 F. and 2000" F. for a time sufficient to form the coating.

7. A method for diffusion coating metallic articles composed of a material selected from the group consisting of iron-base alloys, nickel-base alloys, cobalt-base alloys, and copper-base alloys comprising embedding the article to be coated in particulated, prealloyed charge material consisting essentially of about 30 percent by weight aluminum and the balance substantially all iron and incidental impurities, and heating said charge materials and contained articles in the presence of at least one carrier material selected from the group consisting of ammonium halides and aluminum halides, said heating 14 being conducted with the exclusion of air at a temperature between about 1400 F. and 2000 F. for a time sufficient to form the coating.

8. A method for difiusion coating metallic articles composed of nickel-base alloys comprising embedding the article to be coated in particulated, prealloyed charge material consisting essentially of about 30 percent by weight aluminum and the balance substantially all iron and incidental impurities and heating the charge material and contained articles in the presence of at least one carrier material selected from the group consisting of ammonium halides and aluminum halides, the amount of carrier material present with the charge material being in the relationship of from about 0.25 percent to about 3 percent by weight carrier material to about 97 percent to about 99.75 percent charge material, said heating being conducted with the exclusion of air -at a temperature between about 1400 F. and 2000 F. for a time sufiicient to form the coating.

9. A method for diffusion coating metallic articles composed of cobalt-base alloys comprising embedding the articles to be coated in particulated, prealloyed charge material consisting essentially of about 30 percent by weight aluminum and the balance substantially all iron and incidental impurities, and heating the charge material and contained articles in the presence of at least one carrier material selected from the group consisting of ammonium halides and aluminum halides, the amount of carrier material present with the charge material being in the relationship of from about 0.25 percent to about 3 percent by weight aluminum chloride to from about 97 percent to about 99.75 percent by weight charge material, said heating being conducted with the exclusion of air at a temperature between about 1400 F. and 2000 F. for a time sufiicient to form a coating.

10. A method for diffusion coating metallic articles selected from the group consisting of iron-base, nickelbase, cobalt-base alloys, and copper-base alloys comprising embedding the article to be coated in particulated, prealloyed charge material of a particle size passing a No. 4 mesh sieve and held on a No. 20 mesh sieve, said charge material consisting essentially of from about 19 to about 35 percent by weight aluminum and the balance substantially all at least one metal selected from the group consisting of iron and cobalt and incidental impurities, and heating charge and contained articles in the presence of an aluminum chloride carrier material at a temperature between about 1400 F. and 2000 F. with the exclusion of air for a time sufficient to form a coat- 11. As a charge material for difiusion coating metal articles, a particulated alloy having a composition consisting essentially of from about 19 percent to about 35 percent by weight aluminum and the balance substantially all at least one metal selected firom the group consisting of iron and cobalt and incidental impurities.

12. As a charge material for difiusion coating metal articles, a particulated alloy having a composition consisting essentially of from about 19 percent to about 35 percent by weight aluminum and the balance substantially all iron and incidental impurities.

13. As a charge material for diifusion coating metal articles, a particulated alloy having a composition consisting essentially of about 30 percent by weight aluminum and the balance substantially all iron and incidental impurities.

14. A metallic article having a protective coating of aluminum-containing, oxidation resistant materials formed by heating the article in a particulated, prealloyed charge material consisting essentially of from about 19 precent to about 35 percent by weight aluminum and the balance substantially all at least one metal selected from the group consisting of iron and cobalt and incidental impurities, said heating oonducted in the presence of at least one carrier material selected the group 15 consisting of ammonium halides and aluminum halides at atemperature between about 1400 F. and 2000 F. with the exclusion of air for a time suflicient to form the coating.

15. A metallic article having a protective coating of aluminum-containing, oxidation resistant materials formed by heatin'g'the alticle in a particulated, prealloyed charge material consisting essentially of from about 19 to about 35 percent by weight aluminum and the'balance substantially all iron and incidental impurities, said heating conducted'in the presence of at least one carrier material selected from the group consisting of ammonium halides and aluminum halides in an amount of from about 0.25 percent to about? percent by Weight carrier material to about 97 percent to about 99.75 percent charge material, said heating conducted at a temperature of from about 1400 F. to 2000 F. with the exclusion of air for a time sufiicient-to form the coating. a

16. A metallic article having a protective coating 0 aluminum' containing; oxidation resistant materials formed by heating the article in a particulated; prealloyed charge material consisting essentially of about 30 percent by weight aluminum and the balance substantially all iron and incidental impurities, said heating conducted in the presence of at least one carrier material selected firom the group consisting of ammonium halides and aluminum halides at a temperature between about 1400 F. and 2000" with the exclusion of air for a time sufficient'to form the coating.

References Cited in the file of this patent UNITED STATES PATENTS 2,604,455 Reynolds at al. July 22, 1952 2,657,129 Stern et al. Oct. 27,1953

'- FOREIGN PATENTS 586,241 Great Britain Mar. 12, 1947 812,301 Great Britain Apr. 22, 1959 

14. A METALLIC ARTICLE HAVING A PROTECTIVE COATING OF ALUMINUM-CONTAINING, OXIDATION RESISTANT MATERIALS FORMED BY HEATING THE ARTICLE IN A PARTICULATED, PREALLOYED CHARGE MATERIAL CONSISTING ESSENTIALLY OF FROM ABOUT 19 PERCENT TO ABOUT 35 PERCENT BY WEIGHT ALUMINYM AND THE BALANCE SUBSTANTIALLY ALL AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF IRON AND COBALT AND INCIDENTAL IMPURITIES, AND HEATING CONDUTED IN THE PRESENCE OF AT LEAST ONE CARRIER MATERIAL SELECTED FROM THE GROUP CONSISTING OF AMMONIUM HALIDES AND ALUMINUM HALIDES AT A TEMPERATURE BETWEEN ABOUT 1400*F. AND 2000*F. WITH THE EXCLUSION OF AIR FOR A TIME SUFFICIENT TO FORM THE COATING. 